1. Field of the Invention
The present invention relates to a beam-shaping device, an optical disc device, and a fabrication method of a beam-shaping device, for shaping a beam of light having an elliptic intensity distribution into a beam of light having a circular intensity distribution.
2. Description of the Related Art
Description will be made below on the related art with reference to Japan Patent Laid-Open No. 11-232685 as an illustrative example.
(1) As is illustrated in FIG. 7 which shows a sectional view of the configuration of a beam-shaping device of Japan Patent Laid-Open No. 11-232685, a diffraction plane 2 gives an astigmatism to a beam of laser light emitted from a light source 1 such as a semiconductor laser or the like. Such a beam of laser light becomes equivalent to the beam of light generated from a set of focal lines 4 and 5 (namely, within the xz plane, a beam of light having the focal line 4 as a virtual light emitting point, while within the yz plane, a beam of light having the focal line 5 as another virtual light emitting point). In this connection, the focal line 4 is a focal line extending along the y axis direction, while the focal line 5 is a focal line extending a long the x axis direction.
Since the focal line 4 is closer to the diffraction plane 2 than the light source 1, the beam of laser light deflected by the diffraction plane 2 is expanded along the x axis direction. On the other hand, since the focal line 5 is further away from the diffraction plane 2 than the light source 1, the beam of laser light deflected by the diffraction plane 2 is contracted along the y axis direction.
Furthermore, the beam of laser light is deflected in optical path by a diffraction plane 3 so that the astigmatism is canceled, and becomes equivalent to the beam of light generated from a virtual light emitting point 6. By making the beam of laser light to pass through the two diffraction planes 2 and 3, it is shaped properly in expansion, where the shaping ratio m is given by the expression, m=((L1+L3)/L1)×(L2/(L2+L3)). Here, L0 is the optical path length between the light source 1 and the diffraction plane 2, L1 is the optical path length between the virtual light emitting point 4 and the diffraction plane 2, L2 is the virtual light emitting point 5 and the diffraction plane 2, L3 is the optical path length between the diffraction planes 2 and 3, and L4 is the optical path length between the virtual light emitting point 6 and the diffraction plane 2.
When the diffraction planes 2 and 3 are, for example, constructed with gratings (holograms) having such a sectional shape as shown in FIG. 8, the aberration is generated by the wavelength variation in the light source. By making L2/L0=1.1 to 2.0, however, the aberration generation can be suppressed to a relatively low level.
(2) A portion of the sectional shape of the above described grating shown in FIG. 8 constitutes a staircase form of eight levels and seven steps, and the portion shown in FIG. 8 is periodically repeated to form the actual grating (namely, only one period portion is shown in the figure). Such a staircase form is fabricated by the following three processes of etching the surface of the substrate 2S (or 3S), on which surface the diffraction plane 2 (or 3) is formed; the first etching process (namely, the etching process of removing the portion 7a), the second etching process (namely, the etching process of removing the portion 7b), and the third etching process (namely, the etching process of removing the portion 7c).
By the sectional shape fabricated in a staircase form, the beam of light 8 passing through the substrate 2S (or 3S) of the diffraction plane 2 (or 3) is diffracted to be the diffracted beam of light 9. The pertinent theoretical diffraction efficiency reaches the maximum value of 94.96% when the step height d of one step is given by d=λ/n (λ is the wavelength of the light from the light source, n is the number of levels, and here n=8).
Such conventional beam-shaping devices and fabrication methods thereof (fabrication methods of gratings) as described above have been accompanied by the following problems.
(1) The above described condition (L2/L0=1.1 to 2.0) for cancellation of the aberration generated by the wavelength variation in the light source has been such an inaccurate condition that the condition is only applicable to a limited scope of design conditions. More specifically, the aberration generated by the wavelength variation in the light source cannot be canceled in some cases by the condition of L2/L0=1.1 to 2.0, but can be canceled in some other cases by the conditions other than L2/L0=1.1 to 2.0.
Through the above considerations, the present inventors have noticed that the conventional beam-shaping devices cannot suppress the aberration generated by the wavelength variation in the light source to a sufficiently low level.
(2) In the conventional fabrication methods of gratings, the errors in the precision for positioning of the masks in the individual etching processes lead to the formation of landings in the riser portions. Such a landing is formed, for example, in a midway portions of the deepest riser portion A. The relationship between the landing width Δ and the diffraction efficiency is such as shown in FIG. 9 (calculated on the basis of a step width of w=1 μm), and it can be seen that the diffraction efficiency is decreased drastically with increasing Δ values.
On the other hand, when landings are formed in the midway portions of all the riser portions other than the deepest riser portion A, the diffraction efficiency is calculated to be 96.03% on the basis of the step width of ω=0.1 μm (see FIG. 8). In other words, the generation of the landings in the riser portions other than the deepest riser portion A has an effect to improve, rather than to degrade, the diffraction efficiency. Incidentally, when no landings are formed and all the depths of the riser portions are increased by 5%, then d=λ/8×1.05, ω=0.0 μm, Δ=0.0 μm, and the diffraction efficiency is 94.20%. Thus, when the depth of the riser portion deviates from the optimal condition, the diffraction efficiency is degraded.
Through the above considerations, the present inventors have noticed that the conventional fabrication method of grating applies three times of etching processes to the deepest riser portion A, and accordingly the landing generated in the deepest riser portion A becomes broad in width, which constitutes the main factors causing the degradation of the diffraction efficiency.